Dehydrocyclodimerization is a process in which aliphatic hydrocarbons containing from 2 to 6 carbon atoms per molecule are reacted over a catalyst to produce a high yield of aromatics and hydrogen, with a light ends byproduct and a C2-C4 recycle product. This process is well known and is described in detail in U.S. Pat. Nos. 4,654,455 and 4,746,763 which are incorporated by reference. Typically, the dehydrocyclodimerization reaction is carried out at temperatures in excess of 500° C. (932° F.), using dual functional catalysts containing acidic and dehydrogenation components. The acidic function is usually provided by a zeolite which promotes the oligomerization and aromatization reactions, while a non-noble metal component promotes the dehydrogenation function. One specific example of a suitable catalyst is disclosed in U.S. Pat. No. 4,746,763 and consists of a ZSM-5 type zeolite, gallium and a phosphorus containing alumina as a binder.
The conditions used for the dehydrocyclodimerization reaction result in catalyst deactivation which is believed to be caused by excessive carbon formation (coking) on the catalyst surface. After several days (from about 3 to 10 depending on the operating temperature) enough activity has been lost due to coke deposition that regeneration of the catalyst is necessary. Regeneration involves burning or oxidizing the coke present on the catalyst at elevated temperatures. In addition to loss of activity due to coke formation, catalysts containing a phosphorus modified alumina as a binder are gradually deactivated (over a period of time from several months to about a year) by exposure to hydrogen at temperatures generally greater than 500° C. (932° F.) and particularly greater than 565° C. (1049° F.). This loss of activity due to hydrogen exposure, especially above 500° C. (932° F.), cannot be restored by regeneration means, i.e., burning or oxidation at elevated temperatures. Therefore, the catalyst may also be treated with a fluid comprising water and then dried as in U.S. Pat. No. 6,395,664 B1. As used in this application, regeneration refers to the process of restoring lost activity due to coke formation, while reactivation refers to the process of restoring lost activity due to hydrogen exposure.
Catalyst costs can be significant and extending the usable life of catalysts can amount to large savings. If an operator can use a batch of catalyst for a longer period of time before replacing the catalyst, the operator may experience significant costs savings over time through buying less catalyst. Also, each time the catalyst must cycle through regeneration and reactivation processes costs are incurred. So even with regeneration and reactivation processes, costs are best controlled by also increasing catalyst life. A process is needed which increases catalyst life and may be used in conjunction with known regeneration and reactivation processes. Preferably, the process should be easily incorporated and employed in both existing commercial catalytic dehydrocyclodimerization processes as well as those being designed.
Furthermore, increasing the activity of a catalyst may allow for a lesser quantity of catalyst to be required which in turn allows for a smaller reactor vessel thereby reducing capital and inventory expenditures. On the other hand, increasing the activity of a catalyst may allow for more feed to be processed using the same quantity of catalyst thereby increasing profitability. A process is needed which increases catalyst activity without decreasing selectivity or catalyst life.